add named variables to model
Modified sample and psample to accept named_vars for trace; added example with deterministic
Fixed bugs in named usage; removed call to help() in tutorial example that causes test_examples.py to hang.
Renamed 'named' to 'Deterministic'
One more occurrence of named changed to Deterministic
Merge pull request #325 from pymc-devs/deterministics

Accessing Random variables and named deterministics by name through Model objects